Automatic gain control system



Bec. 27,v 1949 w. M. KELLOGG AUTOMATIC GAIN CONTROL SYSTEM 3Sheets-Sheet 1 Filed Feb. 1, 1946 A QWN A T TORNEV Dec. 27, 1949 w. M.KELLOGG AUTOMATIC GAIN CONTROL SYSTEM 3 Sheets-Sheet 2 JTM WM Filed Feb..1, 1946 AHORA/5y Dec.. 27, i949 w. M. KELLOGG AUTOMATIC GAIN CONTROLSYSTEM 3 Sheets-Sheet 5 Filed Feb. l, 1946 SMH Np n .mi

ATTORNEY -distingishable from -the noise.

Patented Dec. 27, 1949 riep sr:

`-3 Claims. l

.This invention drelates .to signal transmissionzsystemsfandfparticularly to means for controlling @.theenergy. levelatdesiredpcints in a signal trans- ,mission system.

The.inventionisapplicable [for example to a so-called radar .system inwhich recurrent Ypulses Aofradio energy aregtransmitted toward a:targetand lecho .pulses reiiected Vfrom the target rare received-@byaradioreceiver,.meansbeing provided for measuring Athe .delay .periodbetween `.transmitted pulses andcorresponding echopu'lses -receiuediromthe 1targetand thereby indicating therangetoithe target. vIn.measuring the range -.of the ltarget, .itiscormnon -practice to impressathesoutputenergyof v.the radio receiver Yupon a fpair ofgdeectingplatesoia cathode-ray tube .to pieduce rdeflection .of l.the .beam.along one covordinate, a saw-tooth Awave,synchronized with thetransmitted :pulses .being Y,provided .for deiiecting ethelbearn yalong.asecond coordinate. There will .also .be present atv theoutput of the.radio .receiver .Senergy other than :that of the. selected echo signal,.or so-,called noise\.energ-y, which vnormallypro- .duces .1a smaller.deiiection vof .the .cathode-ray beam than the deflection produced bythe se- ,lectedecho `pulses so that the echo pulses are :When fadingoccurs, .it isidesired to increase .the gain of the .radio-receiver so.that .the indicationproduced on lthe `:cathode-rayV tube screen =by.the vecho pulses imaybegobservedlbyfan .operator However, ex-

cessive noise.;appearing .on `the cathode-ray -tube screen.becomesannoying .to .the operator, and .it is, therefore, desirable to.limit :the .gain of the .radio .receiver .to-a certainmaximum .gainsuch `thattlienoise on the .cathode-ray -tube screen will .not becomeexcessive even .if the selected echo. signal completely fades out. y

.Itis desired to.utilize.theecho pulsesignals for `controlling.apparatus other than the range -measuringapparatussuch asapparatus-forcaus- .'ingadirec'ticnal antenna to automatically followamovin g target in .elevation .and azimuth Yso .that thetransmittedpulses will atall tim-es reach and '.he reflected .from thetarget. The pulses .im- 4.pressed upon such .automaticiollowingfapparatus .should prie'ferably'.be .brought V.up-to :a .fixednormal 'level when .fading .occurs .even .under -relatively `weak.signal vconditions such `Vthat .the .radio receiver is operated atiixed maximum gain.

There are, therefore, provided in accordance with .the invention, aspecific .embodiment of which is shown and described herein .iorthepurpose of villustration, a signal transmission system V.comprising ltwo.repeaters for repeating signals f "for controlling `the -gainffof lthesecond 'repeaterto further "control the ,gain of 1 the @system-whilefthe 'rst repeater `is -set'ffor `maximum gain. 'fading of zthesignalaoccurs,-therefore, 'the-gain Vof the repeater tin theradio'receiver Lis'in'crease'd Jupto a predeterminedmaximum andfif' thesig- 4nal "further a'desythej-ga'in fo'f'lthe second-repeater continuesto increase until, Vif complete fading occurs, the Ynoise 'energy at-theoutput of the lsecwond repeateris broughtaup 4tornormal signal -=amplitude The arrangem'ent'fis suchthat Ethere is Asupplied to theutilization apparatus connected to `the -output of theI`second=-1epea.ter all 'the signa-l possible up "to Aaf-desired nnormalamplitude, `without allowing ythe fnoise deflections on thecatl'locle-'ray"tube Yindicatorftobecome so yhigh as 'to beranrioyingt'othepperaton "C`ons'tant-ampl tude energy --at the output of jthensecond repeater will lbe maintained vwhether the AOutput energy isnoise energy 'or signaling energy.

.The inventionwillnow be ldescribed in detail with reference lto theaccoxnpanying drawing, -Figs. v1, '2: and"3 o'f which whenplacedIside-'by side, .in order with Fig. lfatthe l.eft,'constitute aschelmatic View .of agpulse signal *trans-mission system .embodying theinvention.

Referring nowto the drawing, 'there are pro- 'vided an oscillator it#for generating a sinusoidal wave which is irripressed"uponnJ:startinggpulse generatorl -:for generatingra'series of pulses tof briefduration, one for ,each-cycle `ofthe sinusoidal wave. The startingpulses.are impressed .upon a Yradio transmitter |13 which produces a series ofcorresponding brief pulses .of radio .frequency energy. 'Theseradiopulses areimpressed through a suitable couplingarrangement AorT-R ;boxl2 .upon a dipole .directional antenna 'Il fromwhich `the pulses areradiated 'toward Va target. Echo Ypulses reiiectedfrom .the target are`picked upiby .the antenna vIll and .impressed through the `T-R Lboxupon the input of radio receiver `I3 comprising a plurality .ofintermediate "frequency amplifierstages including an ampli'er tube [6.The coupling arrangementllZ serves 'to shuntthe input o'f the .radio.receiver during ,pulse .transmitting `.periods so that .the .sameantenna maybe .used

both for transmission and reception of pulses although separatetransmitting and receiving antennas obviously may be used if desired.The T-R box is a transmit-receive*switch which may be of any desiredtype and may conveniently be a Western Electric Company 709A spacedischarge tube. This tube is essentially a resonant cavity filled withan ionizable gas. During reception with the low voltages of the receivedenergy, the gas is not ionized, the cavity is tuned to resonance and thereceived energy is conducted to the radio receiver I3. During thetransmission of a pulse, the voltages due to the pulse ionize the gas,thus detuning the cavity and preventing the energy of the pulse fromreaching the radio receiver I3.

There is provided a pulse transmitting system comprising space dischargedevices VIO-I, VI02, VIO'3, VIM, VME, Vl'06 and VI I0 for transmittingthe selected echo pulses received from a target to a pulse utilizationapparatus. A circuit comprising resistors RI03 and RIM is connected tothe output of radio receiver I3 so that the echo pulse voltages I8appearing across resistor RIM will be impressed upon the input circuitof tube VIOI. A typical form of the video output of the receiver isshown by I8. The vertical axis shows the voltage amplitude of the targetecho pulses and the horizontal axis shows their time relationship Withrespect to the leakage signal from the transmitter or direct pulse atthe left of the diagram. A pedestal pulse generator I9 is provided forproducing pedestal pulses, one for each cycle of the sinusoidal Wavefrom source I4 impressed thereon through an adjustable phase shifter 20.By adjusting the phase shifter 20, the pedestal pulses may be delayed oradvanced until each echo pulse from a desired target occurs during theVperiod of the corresponding pedestal pulse in order that the desiredecho pulse may be selected or separated from other echo pulses. Thepedestal pulses from the generator I9 are impressed across a circuithaving a resistor RIOI in one of the two parallel branches and, in theother branch path, a condenser CIDI and a resistor I02 in series. Thecontrol grid-cathode circuit of tube VII comprises resistors RI 02 andRIO-4 in series and having their common terminal grounded so that anegative pedestal pulse potential ZI' is impressed upon the control'grid with respect to ground and a positive echo pulse potential isimpressed upon the cathode with respect to ground. A Z50-volt directcurrent source, shown as the battery Il, having its negative terminalgrounded,

' is provided for supplying operating voltages to the screen grid oftube VIIlI through resistors RIIO and'RIll and to the anode of the tubethrough resistors RI I0 and RIOS. In orderl to simplify the drawing,lter condensers used in various parts of the system, such as one in apath connecting the common terminal of resistors RE I 0 and RI 05 toground, are not shown.

The echo pulse and the pedestal are thus Yadded by the circuit of tubeVIOI so that the pedestal and the echo pulse each have a positivepolarity at the anode of the tube. The amplitude of the pedestal isgreater than that of any signal including the selected echo pulse whichis superposed upon the pedestal as shown at 22. The pedestal pulse withthe echo signal superposed is impressed upon the input circuit ofamplier VI02. For this purpose the anode of' tube VIIII is connectedthrough condenser CI03 and resistor Rlll'l to ground, the control gridof tube VI02 being connected to the common terminal of condenser CI03and resistor RIO-I. The cathode of tube VI02 is connected throughresistors RI 08, RIUQ and variable resistor RIIII, all in series, toground, condenser CI02 being connected across resistors RI09 and RI I4.The anode of tube VI02 is connected through a condenser CIUS andresistor RII'I to the cathode of a diode VI03, the anode of the diodebeing connected through resistor RI L8 to ground. Current from thepositive terminal of battery I'I flows through resistors RI I I, RI09and RI Il to ground. Current also ows from the positive battery terminalthrough resistors RI I3 and RI I5 to ground. Condenser C'I is charged tothe voltage across RII3 through a circuit comprising resistors RI I2, RII3 and RI I6. When no anode current is flowing in tube VI02, theanode-cathode voltage is equal to the voltage drop across resistor RlII.Screen grid voltage is applied to tube VIIJZ from source I1 throughresistor Rl l0. The bias voltage across resistors RI'09 and Rl Iii isset so thatrthe pedestal voltage drives the tube VI02 from cut-off to avalue of anode current above which there is adequate operating range tohandle the signal superposed upon the pedestal voltage. When anodecurrent flows, the potential at the anode of tube VI02 is reduced due tothe voltage drop across resistor RI I2 and the condenser CI 06discharges through a circuit comprising resistors RI I2, RI I3 and RII6. Tube Viti? thus acts as a partial clipper and the voltage acrossRIIIS contains the pedestaled echo pulse and the tips of other highamplitude pulses as shown at 23. The resistance of resistor RI I4 may bevaried to control the bias of tube VI02 and thus to control theamplitude of the pedestal appearing at the anode of tube VI0V2. Apedestal of just sufficient amplitude to overcome the bias voltageacross resistor RI I5 permits the pedestaled echo pulse current to flowthrough resistor RI I8 and the diode VI03. The desired echo pulseis'thus completely selected and appears as a voltage across resistor RII8 as shown at 24.

The gain of amplifier stages comprising variable gain tubes VIM and VI05is automatically controlled in response to the Yvariable voltage dropproduced across a resistor RI'50 Which is shunted by a diode VI33, asWill be explained below. The control grid-cathode signal circuit of tubeVIiJll comprises resistor RIIS and condenser C200 to ground and resistorRI02I connecting the cathode to ground. The signal voltage acrossresistor RI I8 is applied to the input circuit o f tube VI04 by means ofthe coupling circuit of resistors RI I8 and RI I9 and condensers CIOIand C200. Screen grid voltage is applied to tube VIM from source I'Ithrough resistor R522 and to the anode through resistor RIZI. The anodeof tube VIM is connected to the control electrode of tube VI05through acondenser Cm3.4 A circuit connecting the control electrode and cathodeof tube VI 05 comprises resistor RIZI and condenser C200 to ground andthence through resistor RI022 to the cathode. Screen grid voltage fortube VI 05 is applied from source I'I through resistor RI22 and anodevolt age is applied thereto through resistor RI23.

The anode-cathode circuit of tube VI05 is coupled to the controlgrid-cathode circuit of tube Vliii through coupling condenser I I0 andresistor Riil, one terminal of this resistor and the cathode of tubeVI06 being grounded. Anode voltage is supplied from source I'I to thistube through escasos resistors R125 andV R1 26 in series-and screen gridvoltage is supplied from'the-common terminal of these resistors. Theanodeof tube Vlfdis connected through a condenser C111 to the controlelectrode of output: amplifier tube -V 1- lila The cathode of tube VHBis connected through resistors Rlld and Ri'32 inV series toground,resistor R1 914 being shunted by condenser 615. Voltage from source 1?is applied to the screen grid of tube V1 1B through resistor R131 andtothe anode of thisl tube through resistors R131 and R133 in series.

The output pulses of tube V1 lilproduced across resistor R132 aresupplied to a desired utilization apparatus, not shown, by Way oi Atl-ieleads 25. These outputpulses maybe used, for example, for controllingthe antenna 1-1- to cause'it to iolll'ow or track` a moving target soth-at the transmitted pulses Will reach the target, be reeeted therefromand picked up by the antenna after reection.

The circuit arrangement for generating the control voltagesV forautomatically controlling the gain of the intermediate frequencyamplifier of the radio receiver 1'3 and o-f lthe ampli-ner stages V104and V185 com-prises the space discharge devices V137, V199, V138, V139,V122, V124 and V123. The anode of tube V1 le is connected through acondenser C11-6 and resistor Ri' to ground, the common terminal oi thecondenser and resistor being connected to the cathode of a diode Vtl.The anode of diode V113? is connected through a circuit comprising aresistor R155 shunted by a condenser C122 to ground`v and is connectedthrough a second circuit comprising a resistorRlSl i-n series withacondenser C123 to ground. The common termina-l of resistor R115?! andcondenser C123 is connected to the control electrode oi"- tube V75-S91When tubeVi i155.5- is quiescent, condenser C1115 is-*charged by currentfrom source 11 through resistors R131, R133' and R163 in series. Whenthe selected pulse of positive polarity is impressed upon the input oftube V111) the anode current increases and condenser C11-5 dischargesthrough a circuit comprising the anode-cathode path of tube V1 1li,condenser() 15E, resistor R132, condenser C122 and the anodecathode pathofi diode V51-M. CondenserC12?` is thereby charged during .each selectedpulse duration with a charge proportional to the pulse lvoltage appliedto the grid of Vile. A4 condenser is used to store .the pulse energy sothat a maximum of energy can be stored Without requiring. excessivepulse voltage amplitudes tov be developed by V1.1.1i. To avoid' build upof voltage .across C1i2-2by accumulation of charge from the .successionof echo pulses corresponding to transmitted pulsasuch that theproportionality oi each. charge to. the selected pulse .amplitude iscontinuously maintained spite .of signal fading. or other erraticchanges'insignal amplitude,.most of the pulse charge on condenser;C122.- is permitted to Ieakoi through resistor Ril'' during. periodsbetween successive pulses. Resistor R157 and condenser G3i-.23 compriseva resistancecapacityl `rilter for suppressing alternating components orthe voltage across. condenser C122' so thatthe voltage across condenser1231-211lv is a relatively'steadyr voltage having an amplitudecorresponding to the average amplitude; taken over a period such asone-tenth second. for example, of theV selected echo pulses duringnormal operation. 'I'hereis providedv a rectifier comprising al doublediode V123, transformerV coupled tof an' alternating current source3`11`fior1supplying current to a load' 6 comprisingpotentiometer R11611`andriiesistorRi 59 :in-series, the positive terminal of the rectier andone" terminal ofl` resistor R159.. being grounded. The supply cir-cuitincludes. a ripple lter com:- prising a series choke coil L1-i1'1 and ashuntconidenser C1121.' and a voltage regulator comprisingv .a seriesresistor Rlfand' a cold cathode, gaseous discharge tube. V124. which.maintains the` unground'ed load. terminal. at substantially 105 voltsnegative withy respect to ground.. The anode of tube VIM is .directlyconnected to' the positive terminal of battery 1'1. The cathode oitubeVl 09 is. connected through' resistors/R152 andv R152 in .series tola terminali ofY resistor` R166. The volt'- .age across resistorszRd 58and R1 52 and the anode- .cathode path of. tube Viet 9', all in series,is there'- tore` 355- Volts. The voltage across potentiometer Riti):and-resistor R159 in series is opposed .to the voltage drop. across`resistors R158 and R162- in series,v thus permitting tube V199 tooperate at a proper bias.

The circuit yor" tube V1 lisser-ves as an impedance transformer. When.the received signal' fades with the resultv that the char-ge oncondenser .C1231 decreases,l the .anode current of tube V-19 increasesand'. thev potential at the cathode of tube Vlvilf will. .become lessnegative-.With respect to ground. The resistance connected inthe cathodepath. of tube VLM is surcently large to cause a change of potential atthe. cathode to be substantially equa-1 to a chang-e of potential attheeon-trol electrode.. Thus the `direct voltage at'the outputof theaveraging circuit which has a hig-h impedance is available acrossthecathode resistance of tube V169- at*` a relatively low impedance. Theresultant voltage measured. between the cathode of tube V109 and' groundis impressed upon a Voltage divider circuit comprising resistors R150,one terminal of which is grounded, and RI-St in series.. A diode V133 isconnected. acrossresistor R150 to provide a shunt path..across-resistor. R156 and thereby to prevent the potential at thecathode of tube Vlilil4 from becoming. positive with respect to ground.The voltage drop across resistor Rl' is applied as a bias voltagetotubeVi 1l!!y by way oi a circuit comprising resistors R14-'8 andRiimlandv tothe tube V by way of resistors R121 and. R1512EA.. Thu-s, assignal. iadesand therefore the charge on condenser C1232 decreases. thevariable gain tubes .V1.04. and V15 are biased. less negativelyy toincrease. the. gain ofA thev amplier.

In order. to restrain the. action. of this automatic controlof amplifiergain. the limi-ting` diode V134 is provided. The limit to which thenegative `bias voltage and.. hence the amplifier gain reducvtion canattain .is determined by diode V-t. The cathode of VIM is connected tothe junction of resistors R 1` and R155' and. the anode is. connected to'the junction oi resistors'RZi and' R2 u1 which` are connected' betweenground and; a nega tive sourcev` of current' such that the anode isnegative withl respect to ground by the voltage drop across RZEB'I;VR261" is adjustable' to permit adjustment of'V the anode'bias voltage.When the negative voltage drop across becomes greatertharr thedropacross R21] 1` the diode V131@ becomes .conductive and of`ioW resistanceas compared' with R151. It acts as a limiter and' preventsv fin'therincrease of the voltage drop across 'Ri1'5ll. The low'liinit'of the.gain ofthe amplifier is thuscontroll'able bymeans/oiRQ'ti. Thisadjustment-serves to' restrain the gain control actions of thev variablegain tubes V51 Ell and V195 in the A`region or" signal level'v Where theautomatic" gain `is active. Ymatic gain control of the intermediatefrequency control of the intermediate frequency amplier AsV will bedescribed later the autocontrolled over a range up to a predeterminedmaximum. This maximum may be adjusted by 'changing the setting of themovable contact of potentiometer R160 which is connected to the cathodeof space discharge tube V108. The control grid of tube V108 is connectedthrough a resistor R154 to the common terminal of resistors R158 andR162. The anode of tube V108 is connected directly to ground and itsscreen grid is connected through a resistor R153 to ground.

The anode of tube V108 is connected through resistor R1024 to thecontrol electrode of a triode V130, the cathode of tube V139 beingconnected to the ungrounded terminal of resistor R159. There is alsoprovided a space discharge tube V122 having its anode and its screengrid connected to the voltage source 11 and having its cathode connectedthrough a resistor R152 to the anode 'of tube V139 and to the controlgrid of tube V122.

The cathode of tube V122 is connected to the screen grid and anode ofone stage 16, at least, of radio frequency amplification of radioreceiver '13 through a resistor 40 and an impedance element 41,respectively.

The intermediate frequency amplifier of the receiver includes a manualgain control not shown in the drawing. This manual gain control enablesadjustment of the cathode bias voltages of the amplier stages andtherefore, in conjunction with the automatic control of the anode andlscreen grid voltages, serves to control theA gainof the receiver. Themaximum anode and screen grid voltage of the intermediate frequencyamplivfier and hence the maximum gain for a given setting of the manualgain control is adjusted by lmeans of the top of potentiometer R160.

If the grid of tube V108 is biased relatively more negatively by movingthe vpotentiometer tap toward ground, the space current of tube V108 isdecreased. As a result the Vcurrent flowing through a portion ofpotentiometer R160 and resistor R159 is increased and the cathode oftube V139 becomes more negative with respect to ground. The spacecurrent of tube V139 flowing from source 1'1 through a circuitcomprising the anode-cathode path of tube V122, resistor R152, theanode-cathode path of tube V139 and resistor R159 is thereforeincreased. The resulting increased voltage drop across resistor R152causes the control grid of tube V122 to become more negative to increasethe impedance of the anode-cathode path of tube V122 and therefore toincrease the voltage drop across the anodecathode path. The potentialsimpressed upon the screen grids and anodes of theA intermediatefrequency amplifier tubes of the radio receiver are therefore reduced tocause'the Again of the radio frequency amplifier to be reduced. Movingvthe tap of potentiometer R160 in the reverse `direction so as to makethe grid of tube V108 less.

negative will cause the maximum Ygain of radio receiver 13 to beincreased.

If fading of the signal, with the accompanying reduction of charge ofcondenser C123, occurs, the potential of the common terminal ofresistors R158 and R162 becomes less negative, thereby causing anincrease of space current through the tube V108. The current flowingthrough resistor R159 is reduced to make the cathode of tube V139 lessnegative and thereby causing a decrease of current through the circuitcomprising resistor R152 and the anode-cathode path of tube V139. Thetube V122 is therefore bia-sed less negatively to decrease the impedanceof the anode-cathode path of tube V122. The Yvoltage applied to theanodes and screen grids of the intermediate frequency amplifier tubes ofthe radio receiver 13 is therefore increased to increase the gain of theradio receiver. Y V

The maximum anode and screen grid voltage is obtained when the signalhas decreased to Va point where the control grid-cathode bias of tubeV108 is equal to zero. If the signal decreases further, the potential atthe common terminal of resistors R158 and R162 will become positive withrespect to the movable tap of potentiometer R160. Y'lube V108 will thendraw grid current with the result that a voltage drop is produced acrossresistor R154 which counteracts the voltage drop across resistor R162and the portion of potentiometer R160 in the grid-cathode circuit. Thegrid bias of tube V108 therefore remains substantially equal to zero andthe gain of the radio receiver remains substantially constant at itsmaximum gain.

When no echo pulse is being received, the noise energy output from theradio receiver will be limited to a maximum amplitude as determined bythe setting of potentiometer R160. However, the gain of the amplifier-comprising tubes V104 and V105 will be further increased after theradio receiver has been brought up to maximum gain. The output energyacross resistor R132 supplied to pulse utilization apparatus is thusautomatically maintained at a substantially constant amplitude. Thisoutput energy will be the recurring echo pulses corresponding to thosereceived from a target or, if the selected pulse signal fadescompletely, the output energy will be noise energy of the samesubstantially constant amplitude. This arrangement permits all thesignal possible to be supplied to the utilization apparatus withoutallowing the noise energy applied to the cathoderay indicator connectedto the output of the radio receiver to become so high as to be annoyingto the operator. Y

What is claimed is:

1. In combination, voltage responsive means for controlling the gain ofa transmission system for transmitting electric signals, comprising anrst space current device having an anode, a cathode and a controlelectrode. a first, a second and a third resistor, a first circuitconnecting said anode and said cathode comprising in series a source ofspace current and said iirst resistor for causing a unidirectionalvoltage to be set up Y said first and second resistors, a second spacecurrent device having an anode, a cathode and a control electrode, afourth circuit connecting the anode and cathode of said second devicecomprising said third resistor and a portion of said second resistor, aiifth circuit connecting the control electrode and cathode of saidsecond device comprising said iii-st resistor and the remaining portionof said second resistor, and means under control of a voltage derivedfrom the anode-cathode circuit of said second device for controllingsaid voltage responsive means.

2. A combination in accordance with claim 1 in which said means forcontrolling said voltage responsive means comprises a fourth resistor, athird and a fourth space current device each having an anode, a cathodeand a control electrode, a sixth circuit comprising in series a sourceof unidirectional current, the anode-cathode paths of said third andfourth devices, respectively, and said third and fourth resistors, aseventh circuit connecting the control electrode and cathode of saidthird device comprising said third resistor, and an eighth circuitconnecting the control electrode and cathode of said fourth devicecomprising said fourth resistor.

3. An electric signal transmission system ccmprising a first portion anda second portion each having an input an an output, a first and a secondgain control means for controlling the gain of said iirst and secondportions, respectively, means for impressing upon the input of saidfirst portion electric energy including recurring pulses which it isdesired to utilize, means connected to the output of said rst portionfor selecting said recurring pulses, means for impressing said selectedl0 pulses upon the input of said second portion, means for setting up arst voltage having an average amplitude proportional to the amplitude ofsaid selected pulses derived from the output of said second portion,means under control of said first voltage for setting up a secondvoltage proportional to said first voltage over a first amplitucle rangeof said first Voltage and having a substantially iixed amplitude foramplitudes of said rst voltage less than a certain predeterminedamplitude, means for utilizing said second Voltage to control said firstgain control means, means under control of said first voltage forsetting up a third voltage proportional to said first voltage over asecond amplitude range of said first voltage and having a substantiallyfixed value for amplitudes of said irst voltage greater than saidcertain predetermined amplitude, and means for utilizing said thirdvoltage to control said second gain control means.

WILLIAM M. KELLOGG.

REFERENCES CITED The following references are of record in the

